The present invention relates to methods for making equipment for diagnostic and therapeutic radiology and, more particularly, to methods for making cathode assemblies used in x-ray generating equipment, such as computerized axial tomography (C.A.T.) scanners. Most particularly, the invention is directed to methods for making x-ray tube cathode structures by preflashing the filaments prior to assembly in the cathode cup, in order to eliminate coiled tungsten filament misalignment or sagging in the cathode cup.
Each x-ray tube is normally enclosed in an oil-filled protective casing. A glass envelope contains a cathode plate, a rotating disk target and a rotor that is part of a motor assembly that spins the target. A stator is provided outside the tube proximate to the rotor and overlapping therewith about two-thirds of the rotor length. The glass envelope is enclosed in an oil-filled lead casing having a window for the x-rays that are generated to escape the tube. The casing in some x-ray tubes may include an expansion vessel, such as a bellows.
X-rays are produced when, in a vacuum, electrons are released, accelerated and then abruptly stopped. This takes place in the x-ray tube. To release electrons, the filament in the tube is heated to incandescence (white heat) by passing an electric current through it. The electrons are accelerated by a high voltage (ranging from about ten thousand to in excess of hundreds of thousands of volts) between the anode (positive) and the cathode (negative) and impinge on the anode, whereby they are abruptly slowed down. The anode, usually referred to as the target, is often of the rotating disc type, so that the electron beam is constantly striking a different point on the anode perimeter. The x-ray tube itself is made of glass, but is enclosed in a protective casing that is filled with oil to absorb the heat produced. High voltages for operating the tube are supplied by a transformer. The alternating current is rectified by means of rectifier tubes (or "valves") in some cases by means of barrier-layered rectifiers.
For therapeutic purposes--e.g., the treatment of tumors, etc.--the x-rays employed are in some cases generated at much higher voltages (over 4,000,000 volts). Also, the rays emitted by radium and artificial radiotropics, as well as electrons, neutrons and other high speed particles (for instance produced by a betatron), are used in radio therapy.
X-ray tube performance can be affected by the alignment of the filament in the cathode assembly. Specifically, during x-ray tube manufacturing, it is important to be able to initially align the filament and have it stay aligned during completion of the manufacturing cycle and during operation of the x-ray tube.
Previously, coiled tungsten filaments used in x-ray tubes were assembled and then aligned in the cathode cup. Once assembled, the filaments were heated to about 2800.degree. C. to produce the desired microstructure. During this heating, when assembled in the cathode cup, many filaments sagged and thus move out of alignment making it necessary to reseat them in the cathode cup and repeat the flashing or the heating to 2800.degree. C. In some instances, this step had to be repeated up to as many as five (5) times until the filament positioned in the cathode cup had both the desired alignment and microstructure.
Because repeating the heating step to produce an aligned filament with the desired microstructure causes increased manufacturing costs, the need to develop new methods for providing a properly aligned coiled tungsten filament in the x-ray tube cathode cup with the desired microstructure became apparent. It has been determined that methods for preflashing and then assembling the filament in the cathode cup having proper alignment with the desired microstructure are desirable. Such methods should provide for a one step assembly process for the coiled tungsten filaments in the x-ray tube cathode cup during the manufacturing process such that the filaments are properly aligned and have the desired microstructure.